1. Field of the Invention
This invention relates to semiconductor memories and more particularly to high performance FET transistors and the method of manufacture thereof.
2. Description of Related Art
In the prior art shown U.S. Pat. No. 5,079,620 of Shur for a "Split-Gate Field Effect Transistor," a higher V.sub.T close to the source region means a smaller voltage swing in the source region for a fixed applied gate voltage, which leads to a higher electric field in the channel region close to the source. Under those circumstances, there is a more rapid increase of electron velocity as a function of distance. Hence, the FET has shorter electron transit time in channel conduction, i.e the FET has a higher speed. However, as the dimensions of transistors shrink, it is difficult to separate the doping of a submicron channel (having a width less than one micrometer) into two controllable regions as described in the Shur patent.
In particular a prior art device similar to some degree to the Shur patent (FIG. 2 thereof) is shown in FIG. 2, a transistor 10 is formed starting with a P-substrate 11 which includes N+ source region 17 and N+ drain region 18. Between the N+ source and drain regions 17 and 18 are regions 15 and 16. Region 15 has a higher threshold voltage than region 18, where the channel region is close to the drain region 18. Above the substrate 11 is a gate oxide layer 12. Above the gate oxide layer 12 and the channel is a gate electrode 14 extending between the source and drain regions 17 and 18. The lower left edge of gate 14 is aligned with the right end of N+ source region 17 in substrate 11. The lower right edge of floating gate 14 overlies and is aligned with the left end of N+ drain region 18.